This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, provides a daily record of Arctic sea ice characteristics for the years 1979 through 2012 derived from passive microwave brightness temperatures. Characteristics include the location of sea ice cover, sea ice age, day of melt onset, and status of melt onset. Data are gridded in the 25 km Equal-Area Scalable Earth Grid (EASE-Grid) 2.0 and provided as netCDF files.

Detailed Data Description

This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, provides a daily record of Arctic sea ice characteristics for the years 1979 through 2012 derived from passive microwave brightness temperatures. Parameters include the location of sea ice cover, sea ice age, day of melt onset, and status of melt onset. Data are gridded in the 25 km Equal-Area Scalable Earth Grid (EASE-Grid) 2.0 and provided as netCDF files.

Format

Data are provided in Network Common Data Form, Version 4 (NetCDF4) format (.nc) following version 1.6 of the Climate and Forecast (CF) metadata conventions. For more information about working with NetCDF formatted data, visit the UCAR Unidata Network Common Data Form Web site.

Grid

EASE-Grid was designed as a versatile format for global-scale gridded data—specifically remotely sensed data—although it has gained popularity as a common gridding scheme for other data as well. Data from various sources can be expressed as digital arrays of varying grid resolutions which are defined in relation to one of three possible projections: Northern and Southern Hemisphere (Lambert's equal-area, azimuthal) and full global (cylindrical, equal-area). For a complete description, visit NSIDC's EASE-Grid 2.0 Format Description page.

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Temporal Coverage

Sea ice cover, day of melt onset, and status of melt onset are available from 1979 to 2012. Sea ice age is only included for the years 1985-2012.

Temporal Resolution

Sea ice cover is available every other day for the years 1979 to 1987, during the SMMR period, and daily from 1988 to 2012. Sea ice ages are updated weekly but provided as a pseudo daily parameter; that is, values in daily NetCDF files remain the same for each seven-day period until new ice ages are available the following week. For example, the value on January 1 is reported for the first seven days of the year (week one), the value on January 8 is reported for the next seven days (week two), and so on. Extra days during leap years are added to week 52.

Refer to Table 3 for a matrix of this data set's temporal coverage and resolution, broken down further by parameter and sensor:

Day of year of melt onset, for cells with melt onset prior to or on file date.

0

No data (or no melt), grid corners

status_of_melt_onset

720 x 720

short (signed)

11

Melt onset begins prior to file date.

12

Melt onset begins on file date.

13

Melt onset begins on a future date.

14

No data or no melt date calculated

0

No data, grid corners

age_of_sea_ice

720 x 720

short (signed)

1-9

Age in years of oldest ice in cell (younger ice or open water may be present).

10

Ten year or older ice

0

No ice age calculated, grid corners

grid_conversions²

720 x 720

short (signed)

21

Land

22

Land mask mismatch assigned to land

31

Sea ice

32

Land mask mismatch assigned to sea ice

41

Open water

42

Land mask mismatch assigned to open water

90

Missing data, grid corners

91

Pole hole

latitude

720 x 720

float (IEEE single-precision floating point)

0° to 90° N

Latitude at center of EASE-Grid 2.0 cell

longitude

720 x 720

float (IEEE single-precision floating point)

-180° E to 180° E

Longitude at center of EASE-Grid 2.0 cell

cols

1 x 720

int

—

x coordinate, center of EASE-Grid 2.0 cell (m from origin)

rows

720 x 1

int

—

y coordinate, center of EASE-Grid 2.0 cell (m from origin)

¹No melt data (filled with a value of 0) before day 61 and after day 245; ²See Land Mask Mismatches for details about this array.

Data files also contain two additional variables which users may find helpful:

coord_system: projection and grid parameters (char)

time: days since 31 December, 1978 (int)

Land Mask Mismatches

Re-gridding to EASE-Grid 2.0 resulted in land type mismatches due to minor differences between the EASE-Grid 2.0 land mask from the Boston University MODIS and the Polar Stereographic Data land mask. The NetCDF variable grid_conversions specifies where the land masks agreed and disagreed after regridding and how mismatches were reconciled. Refer to Table 5 to see how values in grid_conversions correspond to values in the sea_ice_cover variable.

Pixel Type

Value insea_ice_cover

Value ingrid_conversions

BU-MODIS
Land Mask

SSM/I Polar Grid
Land Mask

Land Mask
Comparison

Assigned Value

Table 5. Assigned Value by Pixel Type for Sea Ice, Water, and Land

Land

20

21

Land

Land

Match

—

22

Land

Water

Mismatch

Land

Sea Ice

30

31

Water

Water

Match

—

32

Water

Land

Mismatch

Sea Ice¹

Water

40

41

Water

Water

Match

—

42

Water

Land

Mismatch

Water¹

No Data

90

90

No Data

—

—

—

Pole Hole

91

91

Water

Pole Hole

Mismatch

Pole Hole

¹See New Water Pixels for details.

New Water Pixels

Re-gridding created some new open water pixels between the sea ice and coastline in areas where the ice cover in the original Polar Stereographic grid extended all the way to the coast. To address this issue, the Goddard Space Flight Center Polar Sterographic land mask was regridded to EASE-Grid 2.0 and compared point by point to the BU-Modis land mask. Based on the comparison, the investigators created an intermediate mask to locate and classify disagreements between land masks based on pixel type. Refer to Table 4. The mask is used when creating the grid_conversions, sea_ice_cover, day_of_melt_onset, and status_of_melt_onset NetCDF variables. Refer to the Processing Steps section for additional details.

Note: The mask is static; it is created from land masks and does not change from day to day.

To assess whether or not to reassign new open water pixels as ice, a 3 x 3 pixel neighborhood filter is applied through several iterations. On the first iteration, pixels are reassigned if at least 51 percent of their surrounding eight pixels are ice covered. The second through seventh iterations reassign pixels if at least 33 percent of the surrounding pixels are ice covered. Iterations continue until no more changes are found.

Software and Tools

Data Acquisition and Processing

Data Acquisition Methods

For both melt onset and sea ice age, brightness temperatures were acquired from SMMR, SSM/I, and SSMIS from Nimbus-7 and DMSP F08, F11, F13, and F17 platforms. In addition to these primary data sources, ice age also utilizes: the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) from the AQUA platform for the period 19 June, 2002 to 27 September, 2011; and the NOAA 6-16 platform series for the Advanced Very High Resolution Radiometer (AVHRR) through the end of 2000. AVHRR visible (Channel 1) data is utilized for daytime ice age and infrared (Channels 4 and 5) for nighttime. National Centers for Environmental Prediction (NCEP) re-analysis near surface winds were downloaded via NOAA's FTP site and buoy data were downloaded from the International Arctic Buoy Programme at the University of Washington.

Note that these data are ingested into the algorithm for Sea Ice Motion Vectors (Fowler, 2013), which is then input to the ice age program.

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Derivation Techniques and Algorithms

The sea ice parameter reports the location of sea ice concentrations ≥ 15 percent based on the goddard_merged_seaice_conc parameter in the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration (9 July – 31 July 1987 use the seaice_conc_cdr parameter because the merged data are unavailable). The NOAA/NSIDC climate data record estimates fractional sea ice coverage by combining estimates from two algorithms developed at the NASA Goddard Space Flight Center (GSFC): the NASA Team algorithm (Cavalieri et al., 1984) and the Bootstrap algorithm (Comiso, 1986).

Status of melt onset is created by assigning integer values which indicate whether the cell has a melt onset date prior to the current day (11), on the current day (12), or later in the year (13). Cells with no melt onset date are filled with 0. Refer to Table 4. Melt onset is calculated for days 61-245 of the year.

Day of melt onset corresponds to status of melt onset by providing the day of the year on which past and current melt is observed. In other words, for pixels with status_of_melt_onset equal to 11 or 12, the day on which melt began is recorded in the day_of_melt_onset array.

Ice age is derived from Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors. Using these data as input, ice age is calculated by tracking the ice and comparing adjacent satellite passive microwave images, as well as utilizing wind forcing on the ice and movement of buoys. The oldest ice in each 25 km pixel determines the age which is mapped for any pixel with at least 15 percent ice concentration. If ice from a grid cell is still tracked after the day of minimum ice extent, its age is increased by one year. Thereafter, grid cells that are encountered with ice but have not yet been assigned an age are considered first-year ice (assigned a value of 1).

grid_conversions is discussed in detail in the Land Mask Mismatches section above. This parameter records post-regridding mismatches due to land mask differences between the source data and EASE-Grid 2.0 and is used to create the final sea ice cover data array. The locations of pixel mismatches in grid_conversions (values 32 and 42) point to locations in the sea_ice_cover array that were not directly calculated from the original brightness temperatures and as such should be interpreted with lower confidence.

Processing Steps

The following sections outline the processing steps used to create each NetCDF variable stored data set:

Use mask to locate and fill in melt onset dates for new open water pixels produced by regridding. A filter determines whether each new water pixel needs a melt onset date by examining its eight neighboring pixels. If any neighbors have melt onset dates, the new water pixel is filled with the earliest date. Refer to New Water Pixels.

Run filter to locate cells that change between BU-MODIS and SSM/I Polar grid land or water type due to coastal land/water configuration differences between the land masks;

If cell changed from water to land, keep track of assigned cell land value. Refer to Table 5.

If cell changed from land to water, determine if new water pixel should be filled with sea ice or water using eight pixel neighborhood and keep track of which locations were changed.

First pass: need at least 51 percent of water neighbor pixels equal to ice to convert cell to sea ice, otherwise cell is set to ocean;

Second through seventh passes: need at least 33 percent of water neighbor pixels equal to ice to convert cell to sea ice otherwise cell is set to ocean. Passes are conducted until no changes are found.

Version History

Version 1.2 was released July, 2015. Refer to Table 6 for this data set's version history:

Table 6. Version History

Version

Description

V1.2 (Jul, 2015)

Added 1D arrays named cols and rows that contain x and y coordinates (meters from origin) of the projection. New data files designated v01r02.

Error Sources and Limitations

Cases where land mask mismatches required applying the 3 x 3 pixel neighborhood filter generally occur along the coastline, where it is assumed that the sea ice extends to the coastline. New open water pixels resulted from differences between the source data and EASE-Grid 2.0 land masks in addition to changes in the data due to regridding.

New water pixels which were converted to sea ice are filled with a value of 32 in grid_conversions. Less confidence is associated with these locations because they were not explicitly evaluated for sea ice concentration in the source data. The previously described 3 x 3 pixel neighborhood filter was used to determine whether or not to classify these cells as ice-covered.

Similarly, five iterations of the 3 x 3 pixel neighborhood filter were run to fill the day_of_melt_onset and status_of_melt_onset variables in areas where a melt onset date was not calculated for the early March maximum sea ice extent for each annual snow melt onset grid. For pixels with a value of 32 in the grid_conversions layer, the 3 x 3 filter was applied. If one or more melt onset dates existed within the eight surrounding pixels, the filter selected the earliest melt date.

Error sources for the melt onset dates used to create the day_of_melt_onset and status_of_melt_onset variables include: pixel averaging, sensor errors, and weather effects in the brightness temperature data (Anderson et al. 2014). The Advanced Horizontal Range Algorithm used to calculate melt onset dates for this project has been shown to have an estimated error of about two days, which is consistent with other methods (Drobot and Anderson 2001).

Ice age is computed for the oldest ice in each grid cell, which may or may not reflect the predominant age of ice in the cell. Younger ice or open water may also be present.

The Special Sensor Microwave Imager/Sounder (SSMIS) replaced the SSM/I and became operational in November 2005. The sensor is a passive conically scanning microwave radiometer that measures microwave energy at 24 discrete frequencies from 19 to 183 GHz.

See the following Web pages for additional details about these instruments:

This work was partially supported by NASA grants NNG04GG85G and NNX08AP34A_S02.

Document Information

DOCUMENT CREATION DATE

June 2014

DOCUMENT REVISION DATE

July 2015

No technical references available for this data set.

How To

How do I programmatically access data using spatial and temporal filters?

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